Field of the Invention
The present invention relates to a granular stabilizer for chlorine-containing polymers, which is excellent in the flowability of particles, the dispersibility or kneadability with a resin and the migration resistance in combination, and a process for the preparation thereof.
Tribasic lead sulfate and higher fatty acid lead salts such as lead stearate have heretofore been widely used as thermal stabilizers for chlorine-containing polymers such as vinyl chloride resins. Furthermore, a composite granular stabilizer comprising tribasic lead sulfate and such higher fatty acid lead salt has been proposed as a lead type stabilizer in which dusting or scattering is prevented and the dispersibility into a resin is improved.
For example, in Japanese patent publication No. 3554/66, we proposed a process for the preparation of a granular stabilizer, in which powder of tribasic lead sulfate is homogeneously mixed with a melt of a metal soap such as lead stearate to form a solid solution and the solid solution is molded into granules or small masses. In this process, in order to form this solid solution, it is necessary to use the metal soap in a relatively large amount as compared with the amount of the powdery stabilizer such as tribasic lead sulfate. Although a granular stabilizer of this type can be used for a rigid vinyl chloride resin composition, it cannot be used conveniently for a soft vinyl chloride resin composition. More specifically, if the granular stabilizer is used for a soft vinyl chloride resin composition, kneading with the resin becomes insufficient because of too large an external lubricity and also because of occurrence of migration phenomena such as plate-out and blooming of the metal soap, and therefore, it is practically very difficult or impossible to apply such granular stabilizer to a soft vinyl chloride resin composition.
Japanese patent publication No. 11491/67 discloses a granular stabilizer formed by wet-mixing tribasic lead sulfate with a higher fatty acid lead salt to form a heavy composite material and spray-drying and granulating the composite material. This process is advantageous in that a granular stabilizer can be obtained even if the higher fatty acid lead salt is used in a relatively small amount as compared with the amount of tribasic lead sulfate. However, it has been found that this granular stabilizer is still defective and insufficient in some points.
More specifically, even though the heavy composite material formed by wet-mixing tribasic lead sulfate with a higher fatty acid lead salt takes, in an aqueous medium, a uniform state in which respective particles of tribasic lead sulfate are coated with the fatty acid lead salt, in granules formed by spray-drying this composite material, the fatty acid lead salt is distributed in the state where the fatty acid lead salt separates and rises to the surfaces of the granules. The granular stabilizer having such distribution structure is inevitably poor in the dispersibility or particles of tribasic lead sulfate into a resin. This tendency is also observed in a granular stabilizer which is formed by adding an aqueous solution of a sodium salt of a fatty acid to tribasic lead sulfate formed by reaction of lead oxide with sulfuric acid, to form a coating of a fatty acid lead salt in situe, and spray-drying and granulating the coated tribasic lead sulfate, as taught in Japanese patent publication No. 7537/56.
It is believed that the above-mentioned defects of the known granular stabilizers are owing to the fact that though particles of tribasic lead sulfate are coated with a fatty acid lead salt, the degree of bonding of the fatty acid lead salt to the particles of tribasic lead sulfate is very low and phase separation of the fatty acid lead salt is promoted by heat applied at the spray-drying granulation step and consequent evaporation of water.
We found that when lead oxide, sulfuric acid and a higher fatty acid are reacted at a specific ratio under specific aging conditions, there can be obtained needle crystals of a novel polybasic sulfuric acid/fatty acid lead salt in which the fatty acid component is strongly bonded so that phase separation of the fatty acid component is not caused under spray-drying and granulating conditions, and that if these needle crystals are subjected to spray-drying granulation, rising of the higher fatty acid component to the surfaces of the granules is inhibited and a granular stabilizer for chlorine-containing polymers, which is excellent in the flowability of particles, the dispersibility or kneadability with a resin and the migration resistance in combination, can be obtained.